reflector/refractor equivalence

[chemtom24]

i havent heard of an 8 inch refractor and im pretty sure that it would be costly compared to an 8 inch reflector.

so my question is, is there a sort of equivalence between the two types of scope with different apertures?

could you for instance say that a 100mm refractor will give equivalent views to a 150 reflector? or does aperture win everytime and a bigger reflector will always be better than a small refractor?

[Capricorn]

Really 2 different bits of equipment that happen to perform a similar task.

You also have the problem of differences within the basic options themselves. Spherical reflector or parabolic reflector. In the refractors you have achro, ed and apo.

The "main" difference between to two basic options seems to be that refractors, say, ED and apo, produce a sharper more defined image whereas reflectors can have a bit "softer" image. Basically caused by scattering from the secondary and supports, also relections errors caused by the optical construction.

A big reflector collects light, that is the purpose of it, and that is the plus side of it. If you are not overly bothered about the fainter objects then the choice becomes less critical.

If you are not bothered by CA then get a 6 inch achro, thing is that some cannot abide CA, some don't care.

The point being although bith are telescopes they are equally both different bits of equipment and fulfill a purpose in different way.

[chemtom24]

thanks capricorn

didnt quite get an answer i wanted tho im afraid its kind of hard to get across what im trying to say tho. erm, ok how about this - they say for a reflector that you should only go up to 50x per inch of aperture. this can actually be pushed quite abit further for daytime use but thats a different matter. does this rule apply to refractors too?

aside from physical differences, will a 6 inch reflector be the same as a 6 inch refractor? i think thats what im asking/trying to say

[swamp thing]

IMO the only way to compare them, is by instruments of the same price.

After all that's how you buy them.

No point in trying to compare an 4" Newt and a 4" APO as the APO will be somewhat more pricey. In other words if you can afford a 4" newt it doesn't follow that a 4" APO will be in budget.

You have to look whats available in your budget, then compare.

As Capricorn said there are two many variables in quality and types to make a hard fast rule.

Hope that's diplomatic enough.

Regards Steve

[SiriusB]

A somewhat thorny question & one it's not possible to answer definitively IMO.

There are so many variables to consider:- optical accuracy, coatings, mechanical quality, baffling, focal ratio required, etc etc.

Then there's the question of what are you comparing:- light grasp,resolving power(should be same for both types really,physical laws & all),contrast etc etc. & on what objects?

I'd expect the best 8" refractor money can buy to slightly outperform the best 8" reflector money can buy,(at least on light grasp & contrast) but the cost would be many times as much!!

Probably better to buy a larger reflector for the same money for most purposes?.

From my own observing,(at the cheaper end of the market ) i'd say a smaller refractor gives roughly equivalent views to a slightly larger reflector, but that's probably a gross oversimplification.

[lw24]

Yeah, I've had roughly the same experience as SiriusB; that a slightly smaller refractor performs as well as a slightly larger reflector, e.g: 5 inch Refractor performs as well as a 6 inch Reflector.

There's too many other factors to grant an overall winner.

I'd say this, up to a 5 inch telescope, refractors cost around the same amount as reflectors, and therefore it's probably best to get a refractor in this apeture size. However once you exceed 6 inch or so, refractors become impractical. Most experienced observers will have a small (4 inch) refractor, and a large newtonian (8 inch +), aswell as any other telescopes.

Thats a bit of a generalisation, but thought it may answer your question.

[dweller25]

Roughly, Roughly .....

For planetary contrast - Subtract the size of the secondary from the primary in a reflector to give approx the same size performance is a refractor.

So a 203mm newt with a 47mm secondary has roughly the same performance as a 156mm refractor.

[chemtom24]

what dweller said is pretty much what i was after. tho i was htinking more along the lines of how large an image you get with each type, regardless of what eyepiece you use.

example - my old 130 newt gave a small image of saturn at 200x, whereas my 200 newt gives a much bigger image at 200x.

so im guessing you could say that every 8 inch newt will be pretty much the same with regards to saturn, likewise most 100mm fracs will be the same in image size.

so what i was after was where is the crossover between the fracs and the newts.. ie will a 200 frac give the same size image as a 250 newt at 200x? but then does the newt with its larger aperture give more resolution on the planet, even tho they will appear the same size between the two types?

i think dweller touched on what i mean thanks for all the replies

[dweller25]

Aaaah ....

For the same focal length ie 1000mm newt or refractor the image size will be the same.

At the same magnification image size will be the same.

But the image brightness will be more in the larger scope.

Eg 200mm newt F/5 =1000mm

100mm refractor f/10 = 1000mm

A 10mm eyepiece would give x100 in both scopes but the image in the newt would be brighter

[Mr Spock]

Any scope, no matter what the size, will show Saturn the same size at x200.

If they look different sizes in different scopes, then your magnification calculation isn't quite right.

[glowjet]

The question you raised was I believe covered in the Back Yard Astronomers Guide and pointed out by Dweller, by the time you take off the secondary mirror obstruction your 4" refractor is approaching your 6" reflector, if it is an ED/APO refractor then the resolution is even greater and remember there is no obstruction of the central optical axis. I would imagine a lot of Astronomers are eagerly awaiting the arrival of the new APO refractor range from SW up to 150mm.

[SiriusB]

.... by the time you take off the secondary mirror obstruction your 4" refractor is approaching your 6" reflector, if it is an ED/APO refractor then the resolution is even greater ....

I'd have to disagree there glowjet, Any 4" scope has the same dawes limit as any other 4" scope, any 6" scope will have a 50% lower dawes limit.

Where the apo scores is the usually excellent contrast, often making up for less resolution.

Kinda funny~ me a refractor fan defending the reflector.

Further reading for those interested:

Thoughts, Construction Tips and Musings about Telescopes

[andrew63]

I guess the other thing to consider is say your 4" ED well corrected against a 6" reflector with some coating degradation and collimation less than perfect - and you would be surprised how often this is the case with such equipment - which would give the better view of say saturn?

I know which one I would choose!

andrew

[JamesF]

I'd have to disagree there glowjet, Any 4" scope has the same dawes limit as any other 4" scope, any 6" scope will have a 50% lower dawes limit.

Where the apo scores is the usually excellent contrast, often making up for less resolution.

I agree.

If you have a 4" frac, the "total light gathering area" is going to be around 12.5 square inches. A 6" newt, say, with a 2" central obstruction, you're looking at (assuming I have my maths correct) around 25 square inches. In terms of light gathering capacity the frac is closer to a 5" reflector with the same size obstruction.

(If I've made a pig's ear of the maths, someone point it out, please.)

James

[chemtom24]

4 inch frac 12.5, 5 inch newt 16, 6 inch newt 25, 4 inch newt with 1 inch secondary, 11.7.